A single photon avalanche diode (SPAR) is based on a p-n junction device biased beyond its breakdown region. The high reverse bias voltage generates a sufficient magnitude of electric field such that a single charge carrier introduced into the depletion layer of the device can cause a self-sustaining avalanche via impact ionisation. The avalanche is quenched, either actively or passively to allow the device to be “reset” to detect further photons. The initiating charge carrier can be photo-electrically generated by a single incident photon striking the high field region. It is this feature which gives rise to the name “Single Photon Avalanche Diode”. This single photon detection mode of operation is often referred to as “Geiger Mode”.
U.S. Pat. No. 7,262,402 discloses an imaging device using an array of SPADs for capturing a depth and intensity map of a scene, when the scene is illuminated by an optical pulse.
US 2007/0182949 discloses an arrangement for measuring the distance to an object. The arrangement uses a modulated photonic wave to illuminate the object and an array of SPADE to detect the reflected wave. Various methods of analysis are disclosed to reduce the effects of interference in the reflected wave. In the domain of input devices there is a push towards touch screens and the like. Touch screen technology is fundamentally limited in the accuracy which a user can achieve. For example, the user's finger, stylus, or other touching object will obstruct the user's view of the display thereby reducing the accuracy which a user can achieve. In addition, the mapping between a touch sensitive user interface and a device display for intuitive user operation may use a 1:1 location mapping between a touch sensitive user interface and a device display—this can result in a user interface which is too small to operate accurately.
These issues can be eliminated by locating a touch sensitive user interface such as a track pad at a position not overlaying the screen and hence not implying 1:1 location mapping. However, this uses additional area on the device, rendering it impractical for mobile devices (such as, but not limited to, mobile phones, laptops and tablet PCs) where space is often at a premium.
An additional problem is that the cost of a touch screen and a track pad increase in proportion to their area. This is because the hardware needed to detect a user's input needs to be present across the entire area of the touch screen or track pad.
One attempt to address this issue has been to use virtual projected keyboards which use conventional optics to project the location of a key and to produce a depth map of a scene to determine if a user has interacted with a projected key. However these have not proved successful for a number of reasons.
In particular although conventional 3D cameras are able to produce a depth map of a scene, they are expensive, bulky, have high power consumption and require very high data bandwidth operating at the high frame rates used for low-latency response times essential for user perception of system performance.
The Microsoft Kinect™ system uses light matrix scattering detection techniques to produce a depth map of a scene, but requires heavy data processing, is physically large, and can only achieve a 30 Hz frame rate which compromises accuracy of object tracking. Projection keyboards utilise a similar method of detection but are also limited in frame rate, e.g. Canestra™ Electronic Perception Technology (EPT) keyboard can sense “up to 400 characters per minute” (6.667 per second).
In addition, most widely used touch screen technologies use either components mounted on more than one side of the sensing area, or an overlay for the parent device display which rules out their use for sensing in an area not on a solid surface and restricts their sensing area to an area on the parent device.
As a result of these issues, virtual projected keyboards have been expensive, limited to operating in an imaging mode, have higher bandwidth and processing requirements, and limited due to the need to project onto a hard, non-reflective surface to work.
It is an object of the present invention to overcome at least some of the issues associated with the prior art. It is a still further object of the present invention to provide an alternative to a touch screen as an interface mechanism or input device.